Deep-water turbidites as Holocene earthquake proxies: the Cascadia subduction zone and Northern San Andreas Fault systems

New stratigraphic evidence from the Cascadia margin demonstrates that 13 earthquakes ruptured the margin from
Vancouver Island to at least the California border following the catastrophic eruption of Mount Mazama. These 13 events
have occurred with an average repeat time of ?? 600 years since the first post-Mazama event ?? 7500 years ago. The youngest
event ?? 300 years ago probably coincides with widespread evidence of coastal subsidence and tsunami inundation in buried
marshes along the Cascadia coast. We can extend the Holocene record to at least 9850 years, during which 18 events correlate
along the same region. The pattern of repeat times is consistent with the pattern observed at most (but not all) localities
onshore, strengthening the contention that both were produced by plate-wide earthquakes. We also observe that the
sequence of Holocene events in Cascadia may contain a repeating pattern, a tantalizing look at what may be the long-term
behavior of a major fault system. Over the last ?? 7500 years, the pattern appears to have repeated at least three times, with
the most recent A.D. 1700 event being the third of three events following a long interval of 845 years between events T4
and T5. This long interval is one that is also recognized in many of the coastal records, and may serve as an anchor point
between the offshore and onshore records. Similar stratigraphic records are found in two piston cores and one box core
from Noyo Channel, adjacent to the Northern San Andreas Fault, which show a cyclic record of turbidite beds, with thirty-
one turbidite beds above a Holocene/.Pleistocene faunal «datum». Thus far, we have determined ages for 20 events
including the uppermost 5 events from these cores. The uppermost event returns a «modern» age, which we interpret is
likely the 1906 San Andreas earthquake. The penultimate event returns an intercept age of A.D. 1664 (2 ?? range 1505-
1822). The third event and fourth event are lumped together, as there is no hemipelagic sediment between them. The age
of this event is A.D. 1524 (1445-1664), though we are not certain whether this event represents one event or two. The fifth
event age is A.D. 1204 (1057-1319), and the sixth event age is A.D. 1049 (981-1188). These results are in relatively good
agreement with the onshore work to date, which indicates an age for the penultimate event in the mid-1600 s, the most likely
age for the third event of ?? 1500-1600, and a fourth event ?? 1300. We presently do not have the spatial sampling needed
to test for synchroneity of events along the Northern San Andreas, and thus cannot determine with confidence that the
observed turbidite record is earthquake generated. However, the good agreement in number of events between the onshore
and offshore records suggests that, as in Cascadia, turbidite triggers other than earthquakes appear not to have added significantly
to the turbidite record along the northernmost San Andreas margin during the last ?? 2000 years.